Centrifugal compressors, commonly referred to as “impellers,” are often utilized within auxiliary power units and other types of gas turbine engines to provide a relatively compact means to compress airflow prior to delivery into the engine's combustion chamber. The impeller is typically surrounded by a generally conical or bell-shaped shroud, which helps guide the airflow from the forward section to the aft section of the impeller (commonly referred to as the “inducer” and “exducer” sections, respectively). Certain benefits in impeller performance can be realized by forming one or more ports through the impeller shroud to allow airflow in either of two directions, depending upon the operational conditions of the impeller. In particular, when the impeller is operating near the choke side of its operating characteristic, the ported impeller shroud port in-flows (that is, airflow is drawn into the impeller through the shroud port) to increase the choke side range of the impeller operating characteristic. Conversely, when the impeller is operating near the stall side of its operating characteristic, the ported impeller shroud outflows (that is, airflow is bled from the impeller through the shroud port) to increase the stall side range of the impeller operating characteristic. The airflow extracted from the impeller under outflow conditions may be discharged from the gas turbine engine, utilized as cooling airflow, or possibly redirected back to the inlet of the impeller by a relatively compact recirculation flow pathway for immediate reingestion by the impeller.
While conventional ported impeller shrouds of the type described above can improve impeller performance within limits, further improvements in impeller performance are still desirable. In this regard, it would be desirable to provide embodiments of a ported impeller shroud recirculation system allowing still further improvements in surge margin, range, and other measures of impeller performance. Ideally, such an improved ported impeller shroud recirculation system could be implemented in a relatively low cost, low part count, retrofitable, and straightforward manner and could provide reliable, passive operation. More generally, it would be desirable to provide embodiments of a gas turbine engine or other turbomachine employing such ported impeller shroud recirculation system. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and the foregoing Background.